If it was a bad valve, it would show up when you islolated each cylinder. Its unlikely that more than one went bad at once. Look at your vacuum lines real good with the air cleaner off. There is one in particular, near the intake, that is like a U shaped. This one is known for coming off, and it will make a rough idle and will make ur exhaust note seem like a muscle car at idle.
Im not sure at what it does but its worth looking at, even though im sure you probably did this already. Other than that, it may be a bad fuel distributer. Had one on mine, after weeks of trying to find the culprit. They are somewhat hard to diagnose. If you have any new developments let me know, Id be interested in hearing them. I feel your pain, trying to diagnose a bad idle on a mercedes is a difficult and expensive experience, as it is discouraging.
93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K